Macrophage-derived glutamine boosts satellite cells and muscle regeneration.

Muscle regeneration is sustained by infiltrating macrophages and the consequent activation of satellite cells1-4. Macrophages and satellite cells communicate in different ways1-5, but their metabolic interplay has not been investigated. Here we show, in a mouse model, that muscle injuries and ageing are characterized by intra-tissue restrictions of glutamine. Low levels of glutamine endow macrophages with the metabolic ability to secrete glutamine via enhanced glutamine synthetase (GS) activity, at the expense of glutamine oxidation mediated by glutamate dehydrogenase 1 (GLUD1). Glud1-knockout macrophages display constitutively high GS activity, which prevents glutamine shortages. The uptake of macrophage-derived glutamine by satellite cells through the glutamine transporter SLC1A5 activates mTOR and promotes the proliferation and differentiation of satellite cells. Consequently, macrophage-specific deletion or pharmacological inhibition of GLUD1 improves muscle regeneration and functional recovery in response to acute injury, ischaemia or ageing. Conversely, SLC1A5 blockade in satellite cells or GS inactivation in macrophages negatively affects satellite cell functions and muscle regeneration. These results highlight the metabolic crosstalk between satellite cells and macrophages, in which macrophage-derived glutamine sustains the functions of satellite cells. Thus, the targeting of GLUD1 may offer therapeutic opportunities for the regeneration of injured or aged muscles.

Glutamate dehydrogenase deficiency disrupts glutamate homeostasis in hippocampus and prefrontal cortex and impairs recognition memory.

Glutamate Dehydrogenase 1 (GDH), encoded by the Glud1 gene in rodents, is a mitochondrial enzyme critical for maintaining glutamate homeostasis at the tripartite synapse. Our previous studies indicate that the hippocampus may be particularly vulnerable to GDH deficiency in central nervous system (CNS). Here, we first asked whether mice with a homozygous deletion of Glud1 in CNS (CNS-Glud1 -/- mice) express different levels of glutamate in hippocampus, and found elevated glutamate as well as glutamine in dorsal and ventral hippocampus, and increased glutamine in medial prefrontal cortex (mPFC). l-serine and d-serine, which contribute to glutamate homeostasis and NMDA receptor function, are increased in ventral but not dorsal hippocampus, and in mPFC. Protein expression levels of the GABA synthesis enzyme glutamate decarboxylase (GAD) GAD67 were decreased in the ventral hippocampus as well. Behavioral analysis revealed deficits in visual, spatial and social novelty recognition abilities, which require intact hippocampal-prefrontal cortex circuitry. Finally, hippocampus-dependent contextual fear retrieval was deficient in CNS-Glud1 -/- mice, and c-Fos expression (indicative of neuronal activation) in the CA1 pyramidal layer was reduced immediately following this task. These data point to hippocampal subregion-dependent disruption in glutamate homeostasis and excitatory/inhibitory balance, and to behavioral deficits that support a decline in hippocampal-prefrontal cortex connectivity. Together with our previous data, these findings also point to different patterns of basal and activity-induced hippocampal abnormalities in these mice. In sum, GDH contributes to healthy hippocampal and PFC function; disturbed GDH function is relevant to several psychiatric and neurological disorders.

A metabolomic study of the effect of Candida albicans glutamate dehydrogenase deletion on growth and morphogenesis.

There are two glutamate dehydrogenases in the pathogenic fungus Candida albicans. One is an NAD+-dependent glutamate dehydrogenase (GDH2) and the other is an NADPH-dependent glutamate dehydrogenase (GDH3). These two enzymes are part of the nitrogen and nicotinate/nicotinamide metabolic pathways, which have been identified in our previous studies as potentially playing an important role in C. albicans morphogenesis. In this study, we created single gene knockout mutants of both dehydrogenases in order to investigate whether or not they affect the morphogenesis of C. albicans. The GDH genes were deleted and the phenotypes of the knockout mutants were studied by growth characterisation, metabolomics, isotope labelling experiments, and by quantifying cofactors under various hyphae-inducing conditions. We found that the gdh2/gdh2 mutant was unable to grow on either arginine or proline as a sole carbon and nitrogen source. While the gdh3/gdh3 mutant could grow on these carbon and nitrogen sources, the strain was locked in the yeast morphology in proline-containing medium. We detected different concentrations of ATP, NAD+, NADH, NAPD+, NADPH, as well as 62 other metabolites, and 19 isotopically labelled metabolites between the mutant and the wild-type strains. These differences were associated with 44 known metabolic pathways. It appears that the disequilibrium of cofactors in the gdh3/gdh3 mutant leads to characteristic proline degradation in the central carbon metabolism. The analysis of the gdh2/gdh2 and the gdh3/gdh3 mutants confirmed our hypothesis that redox potential and nitrogen metabolism are related to filament formation and identified these metabolic pathways as potential drug targets to inhibit morphogenesis.

Glutamate Dehydrogenase-Deficient Mice Display Schizophrenia-Like Behavioral Abnormalities and CA1-Specific Hippocampal Dysfunction.

Brain imaging has revealed that the CA1 subregion of the hippocampus is hyperactive in prodromal and diagnosed patients with schizophrenia (SCZ), and that glutamate is a driver of this hyperactivity. Strikingly, mice deficient in the glutamate synthetic enzyme glutaminase have CA1 hypoactivity and a SCZ-resilience profile, implicating glutamate-metabolizing enzymes. To address this further, we examined mice with a brain-wide deficit in the glutamate-metabolizing enzyme glutamate dehydrogenase (GDH), encoded by Glud1, which should lead to glutamate excess due to reduced glutamate metabolism in astrocytes. We found that Glud1-deficient mice have behavioral abnormalities in the 3 SCZ symptom domains, with increased baseline and amphetamine-induced hyperlocomotion as a positive symptom proxy, nest building and social preference as a negative symptom proxy, and reversal/extradimensional set shifting in the water T-maze and contextual fear conditioning as a cognitive symptom proxy. Neuroimaging of cerebral blood volume revealed hippocampal hyperactivity in CA1, which was associated with volume reduction. Parameters of hippocampal synaptic function revealed excess glutamate release and an elevated excitatory/inhibitory balance in CA1. Finally, in a direct clinical correlation using imaging-guided microarray, we found a significant SCZ-associated postmortem reduction in GLUD1 expression in CA1. These findings advance GLUD1 deficiency as a driver of excess hippocampal excitatory transmission and SCZ symptoms, and identify GDH as a target for glutamate modulation pharmacotherapy for SCZ. More broadly, these findings point to the likely involvement of alterations in glutamate metabolism in the pathophysiology of SCZ.

Dysfunctional TCA-Cycle Metabolism in Glutamate Dehydrogenase Deficient Astrocytes.

Astrocytes take up glutamate in the synaptic area subsequent to glutamatergic transmission by the aid of high affinity glutamate transporters. Glutamate is converted to glutamine or metabolized to support intermediary metabolism and energy production. Glutamate dehydrogenase (GDH) and aspartate aminotransferase (AAT) catalyze the reversible reaction between glutamate and α-ketoglutarate, which is the initial step for glutamate to enter TCA cycle metabolism. In contrast to GDH, AAT requires a concomitant interconversion of oxaloacetate and aspartate. We have investigated the role of GDH in astrocyte glutamate and glucose metabolism employing siRNA mediated knock down (KD) of GDH in cultured astrocytes using stable and radioactive isotopes for metabolic mapping. An increased level of aspartate was observed upon exposure to [U-(13) C]glutamate in astrocytes exhibiting reduced GDH activity. (13) C Labeling of aspartate and TCA cycle intermediates confirmed that the increased amount of aspartate is associated with elevated TCA cycle flux from α-ketoglutarate to oxaloacetate, i.e. truncated TCA cycle. (13) C Glucose metabolism was elevated in GDH deficient astrocytes as observed by increased de novo synthesis of aspartate via pyruvate carboxylation. In the absence of glucose, lactate production from glutamate via malic enzyme was lower in GDH deficient astrocytes. In conclusions, our studies reveal that metabolism via GDH serves an important anaplerotic role by adding net carbon to the TCA cycle. A reduction in GDH activity seems to cause the astrocytes to up-regulate activity in pathways involved in maintaining the amount of TCA cycle intermediates such as pyruvate carboxylation as well as utilization of alternate substrates such as branched chain amino acids.

Glucose replaces glutamate as energy substrate to fuel glutamate uptake in glutamate dehydrogenase-deficient astrocytes.

Cultured astrocytes treated with siRNA to knock down glutamate dehydrogenase (GDH) were used to investigate whether this enzyme is important for the utilization of glutamate as an energy substrate. By incubation of these cells in media containing different concentrations of glutamate (range 100-500 µM) in the presence or in the absence of glucose, the metabolism of these substrates was studied by using tritiated glutamate or 2-deoxyglucose as tracers. In addition, the cellular contents of glutamate and ATP were determined. The astrocytes were able to maintain physiological levels of ATP regardless of the expression level of GDH and the incubation condition, indicating a high degree of flexibility with regard to regulatory mechanisms involved in maintaining an adequate energy level in the cells. Glutamate uptake was found to be increased in these cells when exposed to increasing levels of extracellular glutamate independently of the GDH expression level. Moreover, increased intracellular glutamate content was observed in the GDH-deficient cells after a 2-hr incubation in the presence of 100 µM glutamate. It is significant that GDH-deficient cells exhibited an increased utilization of glucose in the presence of 250 and 500 µM glutamate, monitored as an increase in the accumulation of tritiated 2-deoxyglucose-6-phosphate. These findings underscore the importance of the expression level of GDH for the ability to utilize glutamate as an energy source fueling its own energy-requiring uptake.

Differential contribution of the proline and glutamine pathways to glutamate biosynthesis and nitrogen assimilation in yeast lacking glutamate dehydrogenase.

In Saccharomyces cerevisiae, the glutamate dehydrogenase (GDH) enzymes play a pivotal role in glutamate biosynthesis and nitrogen assimilation. It has been proposed that, in GDH-deficient yeast, either the proline utilization (PUT) or the glutamine synthetase-glutamate synthase (GS/GOGAT) pathway serves as the alternative pathway for glutamate production and nitrogen assimilation to the exclusion of the other. Using a gdh-null mutant (gdh1Δ2Δ3Δ), this ambiguity was addressed using a combination of growth studies and pathway-specific enzyme assays on a variety of nitrogen sources (ammonia, glutamine, proline and urea). The GDH-null mutant was viable on all nitrogen sources tested, confirming that alternate pathways for nitrogen assimilation exist in the gdh-null strain. Enzyme assays point to GS/GOGAT as the primary alternative pathway on the preferred nitrogen sources ammonia and glutamine, whereas growth on proline required both the PUT and GS/GOGAT pathways. In contrast, growth on glucose-urea media elicited a decrease in GOGAT activity along with an increase in activity of the PUT pathway specific enzyme Δ(1)-pyrroline-5-carboxylate dehydrogenase (P5CDH). Together, these results suggest the alternative pathway for nitrogen assimilation in strains lacking the preferred GDH-dependent route is nitrogen source dependent and that neither GS/GOGAT nor PUT serves as the sole compensatory pathway.

Delineation of glutamate pathways and secretory responses in pancreatic islets with ß-cell-specific abrogation of the glutamate dehydrogenase.

In pancreatic ß-cells, glutamate dehydrogenase (GDH) modulates insulin secretion, although its function regarding specific secretagogues is unclear. This study investigated the role of GDH using a ß-cell-specific GDH knockout mouse model, called ßGlud1(-/-). The absence of GDH in islets isolated from ßGlud1(-/-) mice resulted in abrogation of insulin release evoked by glutamine combined with 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid or l-leucine. Reintroduction of GDH in ßGlud1(-/-) islets fully restored the secretory response. Regarding glucose stimulation, insulin secretion in islets isolated from ßGlud1(-/-) mice exhibited half of the response measured in control islets. The amplifying pathway, tested at stimulatory glucose concentrations in the presence of KCl and diazoxide, was markedly inhibited in ßGlud1(-/-) islets. On glucose stimulation, net synthesis of glutamate from α-ketoglutarate was impaired in GDH-deficient islets. Accordingly, glucose-induced elevation of glutamate levels observed in control islets was absent in ßGlud1(-/-) islets. Parallel biochemical pathways, namely alanine and aspartate aminotransferases, could not compensate for the lack of GDH. However, the secretory response to glucose was fully restored by the provision of cellular glutamate when ßGlud1(-/-) islets were exposed to dimethyl glutamate. This shows that permissive levels of glutamate are required for the full development of glucose-stimulated insulin secretion and that GDH plays an indispensable role in this process.

Supplement of TCA cycle intermediates protects against high glucose/palmitate-induced INS-1 beta cell death.

The aim of this study is to investigate the effect of mitochondrial metabolism on high glucose/palmitate (HG/PA)-induced INS-1 beta cell death. Long-term treatment of INS-1 cells with HG/PA impaired energy-producing metabolism accompanying with depletion of TCA cycle intermediates. Whereas an inhibitor of carnitine palmitoyl transferase 1 augmented HG/PA-induced INS-1 cell death, stimulators of fatty acid oxidation protected the cells against the HG/PA-induced death. Furthermore, whereas mitochondrial pyruvate carboxylase inhibitor phenylacetic acid augmented HG/PA-induced INS-1 cell death, supplementation of TCA cycle metabolites including leucine/glutamine, methyl succinate/α-ketoisocaproic acid, dimethyl malate, and valeric acid or treatment with a glutamate dehydrogenase activator, aminobicyclo-heptane-2-carboxylic acid (BCH), significantly protected the cells against the HG/PA-induced death. In particular, the mitochondrial tricarboxylate carrier inhibitor, benzene tricarboxylate (BTA), also showed a strong protective effect on the HG/PA-induced INS-1 cell death. Knockdown of glutamate dehydrogenase or tricarboxylate carrier augmented or reduced the HG/PA-induced INS-1 cell death, respectively. Both BCH and BTA restored HG/PA-induced reduction of energy metabolism as well as depletion of TCA intermediates. These data suggest that depletion of the TCA cycle intermediate pool and impaired energy-producing metabolism may play a role in HG/PA-induced cytotoxicity to beta cells and thus, HG/PA-induced beta cell glucolipotoxicity can be protected by nutritional or pharmacological maneuver enhancing anaplerosis or reducing cataplerosis.

Deletion of glutamate dehydrogenase in beta-cells abolishes part of the insulin secretory response not required for glucose homeostasis.

Insulin exocytosis is regulated in pancreatic ss-cells by a cascade of intracellular signals translating glucose levels into corresponding secretory responses. The mitochondrial enzyme glutamate dehydrogenase (GDH) is regarded as a major player in this process, although its abrogation has not been tested yet in animal models. Here, we generated transgenic mice, named betaGlud1(-/-), with ss-cell-specific GDH deletion. Our results show that GDH plays an essential role in the full development of the insulin secretory response. In situ pancreatic perfusion revealed that glucose-stimulated insulin secretion was reduced by 37% in betaGlud1(-/-). Furthermore, isolated islets with either constitutive or acute adenovirus-mediated knock-out of GDH showed a 49 and 38% reduction in glucose-induced insulin release, respectively. Adenovirus-mediated re-expression of GDH in betaGlud1(-/-) islets fully restored glucose-induced insulin release. Thus, GDH appears to account for about 40% of glucose-stimulated insulin secretion and to lack redundant mechanisms. In betaGlud1(-/-) mice, the reduced secretory capacity resulted in lower plasma insulin levels in response to both feeding and glucose load, while body weight gain was preserved. The results demonstrate that GDH is essential for the full development of the secretory response in beta-cells. However, maximal secretory capacity is not required for maintenance of glucose homeostasis in normo-caloric conditions.

Small-interfering-RNA-mediated silencing of human glutamate dehydrogenase induces apoptosis in neuroblastoma cells.

In the nervous system, GDH (glutamate dehydrogenase) is enriched in astrocytes and is important for recycling glutamate, a major excitatory neurotransmitter. The function of hGDH (human GDH) may be important in neurodegenerative diseases such as Parkinson's disease. To test the effect of decreased hGDH expression, several vector-based plasmidlinked hGDH siRNAs (small interfering RNAs) were expressed intracellularly in BE(2)C human neuroblastoma cells. Immunoblotting and reverse-transcription-PCR confirmed that expression of hGDH protein and mRNA was knocked down by co-transfection with phGDH-siRNA vectors in BE(2)C human neuroblastoma cells. TUNEL (terminal uridine deoxynucleotidyl transferase dUTP nick-end labelling) and DNA fragmentation assays 48 h after transfection of phGDH-siRNAs revealed that inhibition of hGDH expression induced cellular apoptosis and activated phospho-ERK1/2 (phospho-extracellular-signal-regulated kinase 1/2). These findings show that inhibition of hGDH expression by siRNA is related to apoptosis in neuronal cells.

[Pitfalls in the diagnosis of congenital hyperinsulinism: a case report and review of the literature]. / Pitfalls bei der Diagnose des kongenitalen Hyperinsulinismus: Ein Fallbericht und Ubersicht der Literatur.

BACKGROUND: Congenital hyperinsulinism is the most common cause for recurrent hypoglycaemia in neonates and infants. Uncontrolled hypoglycaemia leads to seizures and long-term cerebral damage. Often, the diagnosis is delayed because of nonspecific symptoms and confusing laboratory results. PATIENT: We report a patient with hyperinsulinism who was initially wrongly diagnosed as having idiopathic cerebral convulsions and treated accordingly. CONCLUSIONS: Diagnosis of congenital hyperinsulinism is based on a strong suspicion and a thorough family history. Normal random blood glucose or random insulin levels are not helpful in excluding this disease.

Control of the synthesis and subcellular targeting of the two GDH genes products in leaves and stems of Nicotiana plumbaginifolia and Arabidopsis thaliana.

Although the physiological role of the enzyme glutamate dehydrogenase which catalyses in vitro the reversible amination of 2-oxoglutarate to glutamate remains to be elucidated, it is now well established that in higher plants the enzyme preferentially occurs in the mitochondria of phloem companion cells. The Nicotiana plumbaginifolia and Arabidopis thaliana enzyme is encoded by two distinct genes encoding either an alpha- or a beta-subunit. Using antisense plants and mutants impaired in the expression of either of the two genes, we showed that in leaves and stems both the alpha- and beta-subunits are targeted to the mitochondria of the companion cells. In addition, we found in both species that there is a compensatory mechanism up-regulating the expression of the alpha-subunit in the stems when the expression of the beta-subunit is impaired in the leaves, and of the beta-subunit in the leaves when the expression of the alpha-subunit is impaired in the stems. When one of the two genes encoding glutamate dehydrogenase is ectopically expressed, the corresponding protein is targeted to the mitochondria of both leaf and stem parenchyma cells and its production is increased in the companion cells. These results are discussed in relation to the possible signalling and/or physiological function of the enzyme which appears to be coordinated in leaves and stems.

Procyclic Trypanosoma brucei do not use Krebs cycle activity for energy generation.

The importance of a functional Krebs cycle for energy generation in the procyclic stage of Trypanosoma brucei was investigated under physiological conditions during logarithmic phase growth of a pleomorphic parasite strain. Wild type procyclic cells and mutants with targeted deletion of the gene coding for aconitase were derived by synchronous in vitro differentiation from wild type and mutant (Delta aco::NEO/Delta aco::HYG) bloodstream stage parasites, respectively, where aconitase is not expressed and is dispensable. No differences in intracellular levels of glycolytic and Krebs cycle intermediates were found in procyclic wild type and mutant cells, except for citrate that accumulated up to 90-fold in the mutants, confirming the absence of aconitase activity. Surprisingly, deletion of aconitase did not change differentiation nor the growth rate or the intracellular ATP/ADP ratio in those cells. Metabolic studies using radioactively labeled substrates and NMR analysis demonstrated that glucose and proline were not degraded via the Krebs cycle to CO(2). Instead, glucose was degraded to acetate, succinate, and alanine, whereas proline was degraded to succinate. Importantly, there was absolutely no difference in the metabolic products released by wild type and aconitase knockout parasites, and both were for survival strictly dependent on respiration via the mitochondrial electron transport chain. Hence, although the Krebs cycle enzymes are present, procyclic T. brucei do not use Krebs cycle activity for energy generation, but the mitochondrial respiratory chain is essential for survival and growth. We therefore propose a revised model of the energy metabolism of procyclic T. brucei.

Nucleotide sequence of a Porphyromonas gingivalis gene encoding a surface-associated glutamate dehydrogenase and construction of a glutamate dehydrogenase-deficient isogenic mutant.

The nucleotide sequence for a surface-associated protein (A. Joe, A. Yamamoto, and B. C. McBride, Infect. Immun. 61:3294-3303, 1993) of Porphyromonas gingivalis was determined. The structural gene comprises 1,338 bp and codes for a protein of 445 amino acids. The deduced molecular weight of the protein is 49,243. A data base search for homologous proteins revealed significant sequence similarity to the subunit protein of glutamate dehydrogenases (GDHs) isolated from various sources. This protein, which was previously labelled PgAg1, will now be called GDH. Recombinant GDH was purified to homogeneity, and native GDH was partially purified from P. gingivalis. Both preparations exhibited NAD-dependent GDH activity. Intact P. gingivalis and an extract of cell surface components also demonstrated NAD-dependent GDH activity. To help elucidate the role of this protein, an isogenic mutant of P. gingivalis lacking the GDH protein was generated by deletion disruption. Biological characterization of the mutant strain, P. gingivalis E51, demonstrated complete loss of GDH activity. Immunogold bead labelling of intact cells showed that GDH was no longer present on the surface of the bacterial cell. The GDH-negative mutant displayed impaired cell growth, as demonstrated by an increased generation time and an inability to grow to the same cell density as the parent.

Glutamate dehydrogenase deficiency in Machado-Joseph disease.

We studied the activity of glutamate dehydrogenase (GDH) in leukocytes from 23 patients with dominantly inherited ataxia. All the patients were assessed with a rating scale for ataxias and met the clinical criteria for the diagnosis of Machado-Joseph disease. The mean age of onset of symptoms was 37.8, SD 13.4 years and the duration of the disease was 7.4, SD 4.9. Leukocyte GDH activity was significantly decreased (p < 0.001) when compared to 20 normal controls. These data extend previous reports indicating that a GDH deficiency is present in peripheral tissues from some patients with spinocerebellar degenerations. Furthermore, this study suggests that a genetic deficiency of GDH may underlie some forms of dominant ataxias; this deficiency may be marked in patients with Machado-Joseph disease and is not specific for any type of multiple system atrophy.

Glutamate dehydrogenase deficiency in cerebellar degenerations: clinical, biochemical and molecular genetic aspects.

Glutamate dehydrogenase (GDH), an enzyme central to glutamate metabolism, is significantly reduced in patients with heterogenous neurological disorders characterized by multiple system atrophy (MSA) and predominant involvement of the cerebellum and its connections. In human brain, GDH exists in multiple isoforms differing in their isoelectric point and molecular mass. These are differentially reduced in quantity and altered in catalytic activity in patients with clinically distinct forms of MSA, thus suggesting that these GDH isoproteins are under different genetic control. Dysregulation of glutamate metabolism occurs in patients with GDH deficiency and is thought to mediate the disease's neurodegeneration via neuroexcitotoxic mechanisms. This possibility is supported by additional data showing that glutamate binding sites are significantly decreased in cerebellar tissue obtained at autopsy from MSA patients. At the molecular biological level, several cDNAs specific for human GDH have been isolated recently and cloned. Northern blot analysis of various human tissues, including brain, has revealed the presence of multiple GDH-specific mRNAs. In addition, multiple GDH-specific genes are present in humans and these data are consistent with the possibility that the various GDH isoproteins are encoded by different genes. These advances have laid the groundwork for characterizing the human GDH genes and their products in health and disease.

Partially deficient glutamate dehydrogenase activity and attenuated oscillatory potentials in patients with spinocerebellar degeneration.

Glutamate dehydrogenase (GDH, EC 1.4.1.2) catalyzes the synthesis and degradation of glutamate, an excitatory neurotransmitter in the retina. Recently, two forms of GDH, a soluble heat-stable form and a particulate heat-labile form, have been demonstrated to be deficient in some types of spinocerebellar degeneration (SCD). We measured these forms of GDH activity in leukocyte homogenate from patients with SCD (n = 22) and normal subjects (n = 20) who were examined ophthalmoscopically and electrophysiologically. Seven patients with SCD showed attenuated oscillatory potentials (OPs) on electroretinography. The heat-labile GDH activity in these seven patients (78 +/- 51 nmol/mg protein/h) was significantly lower than that of 15 patients with normal OPs (367 +/- 189) and the normal subjects (397 +/- 1720 (P less than 0.001). Our results indicated that patients with SCD could be separated into two groups electrophysiologically, one with normal OPs and one with attenuated OPs. Also indicated was that a deficiency of heat-labile GDH might affect some functions of neural elements in the retina that are responsible for the generation of OPs.